1. Field of the Invention
This invention relates to the field of grinding or comminution and dispersion and more particularly to the reduction of solid matter into fine particles.
The reduction of solid matter into fine powders is a major task of an industrial society. As an example Portland cement is made from finely ground limestone, clay or shale, sand, and coal or other fuel. The limestone, clay or shale, and sand are subjected to a thermal process in which the heat is derived from the coal and the results are clinkers of material that must again be ground to produce the cement. Gypsum, after claching, must be ground to produce sheet rock and other such products.
The food industry grind many products including wheat, corn, rice, spices, sugar, and even chocolate. Paints, inks, and so forth use ground pigments and in turn undergo a dispersion process to disperse the ground pigment in a suitable vehicle.
Ceramics are made from finely ground materials. Generally the better the grind the better the ceramic product. Metals are ground as part of powder metallurgy and to prepare metallic pigments.
2. Description of the Prior Art
One of the oldest and simplest methods of grinding materials to fine powders uses a ball mill that generally consists of a horizontal cylindrical chamber that may be of any size. Ball mills have been constructed in sizes of up to eighteen feet in diameter by thirty or forth feet long.
For many applications that ball mill is about half full of steel or ceramic balls in addition to the material to be ground. The balls roll over one another and aid in the grinding process.
In cases where the material to be ground, such as a paint, consists of a fine pigment to be dispersed, the balls, usually called the grinding media, are essential to the process while in other cases such as the grinding of cement clinker, the media is omitted. In this latter case the larger clinkers act as media for the smaller ones and a means is usually provided for extracting only the finer particles from the mill.
While the ball mill is effective and reliable, it tends to be large and slow. The physical size of the mill tends to cause it to be high in capital cost for the amount of work done.
There is much art having to do with overcoming the deficiencies of the ball mill. Alternative approaches to the task of grinding include mills wherein a material is stirred with media by means of mandrels. In another approach the material being ground in a liquid carrier is subjected to high shear rates by high speed blades or by being forced through narrow gaps between rapidly moving surfaces. These devices are most useful for dispersion while a ball mill both grinds and disperses.
In yet other attempts to obtain the benefits of a ball mill while overcoming its deficiencies, considerable prior art has addressed planetary mills in which the grinding chamber is orbited about an axis parallel to the axis of the grinding chamber. In such art the planetary motion imparts a centrifugal force that aids the action in the grinding chamber.
For example, Wegmann in U.S. Pat. No. 405,810 discloses an orbiting mill driven by planetary gears so as to produce centrifugal forces to aid in the grinding. While well adapted to a batch process, the Wegmann device precludes direct attachment of input and output tubes.
Another form of planetary mill is disclosed in Pendleton, U.S. Pat. No. 458,662. Due to the necessary supporting framework, the feed to the mill is required to pass through the machine axis by means of a rotating seal. A similar rotating seal feed is required by Hertzfeld in U.S. Pat. No. 569,828 which describes vertically oriented orbiting grinding chamber.
West in U.S. Pat. No. 1,144,272 describes a multiple grinding tube centrifugal mill having a planetary gear drive. Access to the grinding tubes is provided by doors on the side of the tubes thus permitting only batch operating processes. Eppers in U.S. Pat. No. 1,951,823, like Pendleton and Hertzfeld above, discloses an axial feed with rotating seals for a planetary mill.
Matthews in U.S. Pat. No. 2,209,344 discloses a planetary rock crusher employing bars as the grinding media. Ore is fed into the mill at its axis and then circulated through the several planetary grinding chambers. Shideler et al., in U.S. Pat. No. 2,387,095, discloses a multiple tube planetary polishing or abrading machine equipped with a fixed eccentric to maintain the tubes in a fixed orientation relative to ground. The tubes are individually closed with stoppers to permit only batch operation.
Limb, in U.S. Pat. No. 2,874,911, discloses a planetary ball mill wherein the grinding chamber rotates more rapidly about its own axis than it does about its orbital axis, thereby limiting its operation to a batch process.
Joisel, in U.S. Pat. No. 2,937,814, discloses a planetary ball mill having two forms. The first form describes a discontinuous or batch process while the second form shows a continuous grinding process using rotary seals at the inlet and outlet. Separate motors are provided to drive the mixing chambers and to provide the orbital motion so that the relative speeds may be varied.
Matsunga et al., in U.S. Pat. No. 3,513,604, discloses a planetary polishing machine having a variable ratio of the orbital rotation rate to grinding chamber rotational rate so as to necessitate a batch process. Wilkinson, Jr., in U.S. Pat. No. 3,529,780, discloses a continuously fed planetary grinding mill having an intermittent discharge into a curved trough.
Bloch, in U.S. Pat. No. 3,876,160, describes a centrifugal mill wherein the grinding chambers and orbital motion are separately controlled. Alternatively, the grinding chamber axis may be parallel to the orbital axis, perpendicularly to the orbital axis, or varied respective to the radius of the planetary movement. However, in all cases a batch operation is contemplated.
Freedman et al., in U.S. Pat. No. 3,190,568, discloses a cell disintegrating apparatus for batch or continuous operation in which the disintegrating tube is rapidly oscillated in a direction perpendicular to its axis to shake and abrade cellular material. Ohno, in U.S. Pat. No. 4,057,191, discloses a grinding mill having a grinding tube with one end mounted in a spherical bearing while the other end is moved in a circular path by a rotating crank so as to make a hybrid form of planetary mill.
German patent 260,777 (1913) discloses a planetary ball mill having a toroid-shaped grinding chamber wherein the grinding chamber radius is significantly greater than the planetary or orbital motion radius. Both the feed and discharge are gravity controlled.
German patent 1,097,790 (1961) discloses a planetary crushing mill for granulated solid material suspended in a liquid wherein the rotation of the grinding chamber is independent of the planetary or orbital motion, thus requiring rotating seals at the inlet and outlet of the grinding chamber.
British patent 593,777 (1947) to Benham discloses a planetary grinding mill in which a spheroidal grinding chamber is orbited about a vertical axis and simultaneously oscillated about its own axis. The spheroidal grinding chamber is supported from one end in a pivoted bearing so that variations in the centrifugal force due to the quantity of material being ground can be used to control the feed rate to the grinding chamber. Material exits the grinding chamber through a mesh screen so as to provide control over the size of grind. In order to make the classification scheme work properly it is essential to the Benham patent that the grinding chamber have a vertical axis.
Canadian patent 1,089,428 (1980) discloses a planetary grinding mill driven by a pair of eccentrics such that the orbiting radius is small compared with the grinding chamber radius so as to provide "a continuous operation in the sub critical range at a high grinding rate per unit of volume of the grinding drum . . . " (page 1, 11. 22-23).
French patent 1,088,571 (1955) discloses a planetary grinding mill driven by pairs of counterbalanced cranks connected by a pair of crossarms to which one or more grinding chambers are affixed.
In U.S. Pat. No. 5,029,760 I describe a system wherein a rotatable drum assembly carries two rotatably mounted grinding tubes that are constrained to have no net rotation with respect to the base of the machine. Access to the grinding tubes may be made only to their ends and only one of the two grinding tubes may be addressed from either end. A second embodiment uses a series of four rotating wheels that drive two oppositely mounted frames that each carry one or more grinding tubes.
In U.S. Pat. No. 5,205,499 I describe a rotatable drum assembly that carries a single grinding tube that has advantages of permitting access to the grinding tube from both ends.